The power requirement of electrical loads in motor vehicles is rising sharply due to the increasing number of comfort and convenience loads and the electrification of auxiliary systems. In certain operating states, the electric power made available by the generator may no longer be sufficient to cover the entire power requirement. To be able to initiate selective measures within the framework of an electrical energy and load management, information may be required concerning the instantaneous relationship of power supply and demand.
When using safety-critical electrical loads, such as electromechanical brakes (EMB), in a motor-vehicle electrical system, the energy supply of this system must be monitored constantly in order to be able to recognize a malfunction in a timely manner and initiate countermeasures. For this purpose, modern vehicle electrical systems may include an energy and electrical load management system, by which it may be possible to check the vehicle battery regarding its state of health prior to switching on a load. If there is danger of exceeding certain load limits of the battery, either measures may be carried out for improving the state of health of the battery, for example, by increasing the charging voltage, or measures may be carried out for reducing the battery load, particularly the switching-off or dimming of electrical loads.
A motor-vehicle electrical system may be supplied by a battery and, when the engine is running, by a generator. The load capacity or state of health of a vehicle battery may be assessed using suitable battery models, making it possible to ascertain whether certain electrical loads may be switched on, or whether load-reducing measures must be initiated to prevent a failure of safety-critical devices. Such a battery model is described, for example, in DE-P 101 065 08.6. However, in that case, the power made available by the generator is not taken into account.
It may be that the degree of generator utilization may be ascertained via a so-called DF-signal (control signal by which the excitation of the generator is activated or deactivated). However, in this context, a conclusion about the instantaneous power output, and, above all, the instantaneous power reserve of the generator is not possible, since the power output of the generator is strongly dependent on the specific operating state (temperature, voltage, rotational speed, capacity utilization).